This invention relates to a protective circuit which includes an electronic switching amplifier in operative association with an electromagnet.
The invention relates more particularly to such a circuit having an amplifier which turns on the electromagnet with timing pulses, a cutoff voltage limiting circuit being connected between the amplifier and voltage limiting circuit.
Electromagnetically actuated positioning and actuating elements are frequently provided on industrial sewing machines for semi-automatic operation, for the initiation, performance, or termination of certain automated work steps; for example as described in U.S. Pat. No. 3,211,117 for electromagnetically actuated thread cutters on sewing machines. Moreover, the actuation of electromechanical devices of other types, in machine tools or assembly lines for example, is often accomplished electromechanically. In other words, an electrically or electronically controllable electromagnet is used as a positioning element in various applications.
It is known to control such electromagnets by providing continuous holding energization during the switched-on interval of the electromagnet by using timing pulses. A mean effective energization is produced in such holding energization by timing pulses as a function of the predetermined pulse duty factor and the given amplitude. However, in order to energize the electromagnet rapidly, the latter is overenergized briefly at the start of operation by applying the supply voltage continuously at the above mentioned constant amplitude over a certain period of time and thereafter switching over to continuous operation with timing pulses; in other words the holding operation is affected with reduced magnet energization, only after the electromagnet has initially responded.
The problem is that during this continuous pulsed operation, periodic cutoff voltages appear during the interval that the electromagnet is connected to the switching amplifier, which is connected in series with the electromagnet. This is known to increase directly with the steepness of the switching characteristic of the switching amplifier as it shuts off. On the other hand, it is desirable to produce a switching characteristic which is sufficiently steep to be able to bypass the switching amplifier with ideal switching operation, practically avoiding the appearance of internal power losses. In order to prevent the quite considerable cutoff voltage peaks which appear periodically from breaking through the high-resistance collector-emitter path of the open switching amplifier, it is conventional to use cutoff voltage limiting circuits as protective circuits. The effect of these protective circuits is to limit the voltage peaks to the blocking voltage which is permissible depending on the individual switching amplifier. Cutoff voltage limiter circuits of this type consist as a rule either of a feedback path from the collector to the base of the input transistor of the switching amplifier via a Zener diode which is, as a rule, in the form of a Darlington circuit, or of a series circuit connected in parallel with the winding of the electromagnet, the series circuit being composed of a free-running diode and a series resistor.
The first of these conventionally employed limiting circuits, wherein the forward voltage of the Zener diode (which in practice is further connected in series with a reference voltage diode) depending on the permissible blocking voltage for the switching amplifier, has the disadvantage that when this permissible blocking voltage is exceeded by the cutoff voltage peaks, the switching amplifier is temporarily driven in the forward direction. It then consequently operates in an unfavorable power loss range of its characteristic, the energy must be dissipated in the cutoff process being converted to heat losses. Especially in modern commercial Darlington circuits provided with plastic housings, these cutoff heat losses can cause the destruction of this transistorized circuit, which constitutes the switching amplifier.
The second alternative for conventionally employed limiting circuits, wherein the cutoff voltage peaks are practically shortcircuited through the free-running diode, has the disadvantage that the series resistor which determines the response threshhold converts all of the energy into heat losses, so that, especially in circuits constructed in tight spaces, inadmissible radiated heat losses will be generated by this resistor.
German Auslegeschrift (Published Patent Application) No. 2,049,179 teaches a protective circuit for a reversible drive motor, whose armature circuit is connected in parallel through the reversing switch to a free-running diode in the form of a controllable electronic valve (switch). In order to eliminate electrical and mechanical overloads on the drive motor in the event of inadmissible actuation of the reversing switch, especially when the motor is disconnected from the line, in such manner that this valve (switch) will be opened no earlier than at this particular time.